Previously we had reported the presence of insulin in extracts of rat brain and other extrapancreatic tissue. Although the synthesis of extrapancreatic insulin is unknown, its concentration in some tissue, e.g. brain, testes, is independent of insulin concentration in blood. Presently, we have detected insulin mRNA in liver, testes and brain subsections using reverse transcriptase and polymerase chain reaction (RT-PCR). In in situ hybridization histochemistry on brain tissue slices using an antisense oligonucleotide probe there was an accumulation of granules in discrete brain regions. These data suggest that insulin in brain and other extrapancreatic tissues may be of local origin.